Thursday, December 25, 2008

pouring water (curing) of concrete after it been poured

What is the name given for the procedure of pouring water on concrete?
its called curing, a processed where concrete is left moist due to the exothermic reaction that takes place which may give cracks on the concrete.

Why Cure?
The major objective of concrete curing applications is to prevent the rapid loss of water from the concrete. As concrete loses water due to evaporation from the top surface, differential drying shrinkage can occur. This is a major contributor to shrinkage cracking. The application of curing methods reduces the loss of water from the surface of the concrete. It also permits more complete hydration of cement in the concrete itself. Minimizing evaporation also helps control the temperature of the concrete during its early-age stage.

Curing operations should begin after the water sheen disappears from the surface, and after any texturing operations have been completed. In the case of a curing compound, the membrane formed by the compound should not be disturbed after it is placed.

Rapid Drying Conditions
In rapid drying conditions, a light water fog may be necessary to maintain moist surface conditions prior to the application of curing methods. Light water fogging can be accomplished during a short period of time when the concrete surface begins to dry but before the curing operations can begin, such as prior to texturing operations have been completed.

Use of Ambient Weather Condition Information
Ambient weather conditions, such as wind speed, relative humidity, and air temperature can interact with the temperature of the concrete to cause excessive water evaporation from the concrete surface. Since different curing methods provide different levels of protection, knowing the amount of protection required is important in determining the method to use. In order to know the required level of protection, the ambient conditions and concrete temperature must be known. A portable weather station that records the ambient conditions and automatically predicts evaporative water from the concrete surface can be an invaluable tool for controlling water loss from the concrete surface. Such a tool can also warn in advance when conditions approach predefined limits of evaporation.
Curing Methods
Various concrete curing methods are available, and each provides different levels of protection. A single coat of liquid curing compound generally provides the least protection, but additional coats can improve its performance. Polyethylene sheets, cotton mats and wet burlap provide additional protection.

Liquid Curing Compound

White-pigmented, liquid membrane curing compound is used most often due to its low cost and ease of application. It does not require great amounts of labor, nor does it expensive, bulky material, such as cotton mats. It's disadvantages are that it provides the least amount of protection, and the membrane can be ruptured inadvertently.

The liquid curing compound should be white, to avoid excess heat absorption from the sun (Figure 4.1). Also, the white color enables construction workers to check more easily for coverage uniformity and gaps in the coverage. The liquid compound must be constantly agitated during application to ensure that the mixture is applied correctly. The curing compound spraying operation should be shielded from the wind throughout the process.

The compound must cover all exposed surfaces, including the sides of the pavement slab. The compound should not, however, be applied into any joints in the pavement. For ultra-thin whitetopping, curing compound should be applied at twice the normal application rate, due to its extra sensitivity to drying shrinkage.

Figure 4.1 - White-Pigmented, Liquid Membrane Curing Compound.

Plastic or Waterproof Paper

Plastic, or polyethylene, sheeting provides good protection to the concrete from water evaporation from the surface (Figure 4.2). It requires more labor than liquid curing compound, yet it is not as bulky as cotton mats or burlap. Waterproof paper may also be used in the same manner described here for plastic sheeting, but is not as common.

The plastic sheeting must not have any rips or tears through which water can escape. The sheets should overlap to provide full coverage for the concrete surface. Just as with curing compound, the sheeting should cover all exposed concrete surfaces, including the edges of the pavement slab. Active methods must be used to hold the sheeting in place. Do not assume that they will remain in place of their own accord.

Figure 4.2 - Polyethylene Sheeting Used as a Curing Method.

Cotton Mats or Burlap

Cotton mats represent a great increase in evaporation protection, both by providing additional moisture if needed, and by protecting the concrete from ambient conditions such as low humidity, high wind speeds, and high temperatures. Cotton mats and wet burlap must be kept continually moist. When the mats get dry they can become more harmful than without them due to "wicking" action which draws moisture from the concrete into the mat.

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